1. Field of the Invention
The present invention relates to the art of the production of hydrocarbons from subterranean formations and, more specifically, to blocking gels of the type used in well bore operations for the production of hydrocarbons.
2. Description of the Prior Art
Occasionally, production from well bore operations must cease temporarily to perform auxiliary procedures such as repairs at different depths of a subterranean formation. The repairs are called workover operations. Workover operations frequently use heavy brines and other fluids to maintain pressure control within the reservoir. the fluids can leak-off into the production zone, causing damage which interferes with the efficient operation of the well.
Isolating the production zone, however, protects it from damage. Specific blocking materials, such as solid blocking agents or temporary blocking gels isolate the production formation. The solid blocking agents, for example Nylon or rubber balls, are injected into the fluid stream and seal the production formation by physically stopping up perforations in the formation. When the injection ends, the material is no longer held against the perforations and falls to the bottom of the well.
The use of temporary blocking gels successfully protects the production zone. Blocking gels formed by gelation of suitable polymers, such as appropriate polysaccharides produce a relatively impermeable barrier across the production formation. The barrier cordons off the production zone from the area undergoing the workover operations. These areas must remain separated until production is ready to resume.
Production resumes after removal of the blocking gel. The recovery of the blocking gel is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion is referred to as "breaking" and is often accomplished by incorporating chemical agents, referred to as breakers, into the initial gel. Blocking gels have been used in low temperature zones for many years. However, certain problems have been associated with isolating high temperature subterranean zones which were mostly due to the insufficient or unstable rheological properties of the crosslinked blocking gels.
The present invention has as its object to provide an improved blocking gel which is suitable for use from low to relatively high temperatures greater than about 250.degree. F. to enable temporary isolation of the producing zone for an extended period of time.
Another object of the invention is to provide such a blocking gel which can be removed at any time without leaving damaging polymeric residue in the subterranean formation.
Another object of the invention is to provide a high temperature blocking gel for temporary workover operations which utilizes a guar polymer which is more temperature stable and which provides improved rheological characteristics over previously known cellulose polymer blocking gel systems.